The invention described herein was made in the performance of work under a NASA contract and by an employee of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. xc2xa7202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. xc2xa7202, the contractor elected not to retain title.
The present invention relates to cryogenics, and more particularly, to testing of thermal insulation materials for cryogenic systems.
Cryogenics is fundamentally about energy, and thermal insulation is about energy conservation. The technological developments of the past century have led to insulation systems that have approached the ultimate limit of performance. More technologies and markets forecast for rapid expansion into the 21st century will require, in many cases, not superinsulations but more efficient systems for a wide variety of cryogenic applications. Although bulk storage and delivery of cryogens such as liquid nitrogen, argon, oxygen, hydrogen, and helium are routinely accomplished, cryogenics is still considered a specialty. Superior methods of thermal insulation are needed.
Thermodynamics is essentially about money and is a tradeoff between refrigeration (energy bill) and the refrigerator (capital cost). In addition to the energy required to liquefy the gases, much energy is expended in the extraction or separation of these desired gases. Any product losses during storage and transfer can therefore be directly equated to monetary losses. The wide-scale proliferation of nitrogen and carbon dioxide as refrigerants is dependent on low-cost production, distribution, storage, and end-use application systems.
Cryogenic insulation is a very specialized insulation which requires very special properties. As opposed to usual insulation, cryogenic insulation must be capable of operating at very low temperatures, i.e. cryogenic temperatures between about xe2x88x92130xc2x0 F. and xe2x88x92450xc2x0 F., while retaining functionality, especially flexibility, at those temperatures.
Standard multi-layer insulation (MLI) systems, such as those using aluminum foil and fiberglass paper spacers, represent the benchmark for comparison. MLI or superinsulation requires a vacuum level below 10xe2x88x924 torr to be effective. Other drawbacks of MLI are that it is highly anisotropic, is sensitive to compressive loads and edge effects, requires careful attention during installation, and is often limited in application by awkward structural complexities. Furthermore, the steps of evacuation, heating, and vacuum retention are costly and time consuming. Thermal performance degrades rapidly for vacuum levels above 10xe2x88x923 torr.
It is important to recognize that there are three levels of thermal performance: ideal, laboratory, and industrial. Industrial (or actual) performance is typically several times worse than the laboratory performance and often 10 times worse than the ideal. The heat leak for the overall mechanical system can in turn be several times more than that estimated for the insulation system alone.
The appropriate choice of a thermal insulation system depends on matching the performance level with the overall cost. That is, the performance must justify the cost. The actual operating conditions must first be considered. An analysis of the total heat leak of the mechanical system is needed to determine the insulation requirements. Often only a common sense thermal review of the system is needed to ascertain which level of insulation material should be selected. The performance level will dictate the insulation materials and mechanical support structures or joining devices to be used.
The main factors to consider are: (1) operating conditions of the system, (2) total heat leak of the mechanical system, (3) material properties such as density and compatibility, and (4) method of testing and evaluation. Attention should also be given to offering advantages such as easier installation, maintenance, and modification where possible.
Testing of such thermal insulation materials is known. One method is the cryogenic liquid boil-off technique. The basic cryogenic liquid boil-off method is simple in concept but extremely difficult in practice. Thermal guards to reduce unwanted heat leaks to tolerable levels are required. The test articles are typically installed as blankets.
Existing boil-off apparatuses for cryogenic insulation testing are common, but few are in operation because of the extreme difficulty in obtaining accurate measurements. Many, and perhaps most, of these devices are not designed for direct thermal performance measurement and thus offer only xe2x80x9ccalculatedxe2x80x9d or xe2x80x9ccomparativexe2x80x9d or xe2x80x9cestimatedxe2x80x9d or xe2x80x9cperformancexe2x80x9d k-values. Set up times are typically very lengthy. Testing of continuously rolled products (which are most commonly used) is not possible. Measurement of temperature profiles is either not done or is minimal because of the practical difficulties associated with the placement, feed-through, and calibration of the sensors. Vacuum levels are usually restricted to one or two set points or are not actively controlled.
Thus, reliable, accurate, repeatable, and reasonable methods of testing a variety of insulation materials are desirable. The testing to obtain the necessary thermal performance and vacuum performance characteristics must be practical from the engineering point of view. A true (that is, quantitative and scientific) apparent thermal conductivity measurement (k-value) for a material system under a certain vacuum pressure level and a given pair of upper and lower boundary temperatures is needed.
More specifically, testing large size prototype material systems in a typical actual-use configuration is needed. The ability to test continuously rolled insulation materials (that is, not blanket form) is desirable because other forms such as seamed blankets will drastically affect the test results, thus giving totally inaccurate readings in most cases. The ability to quickly change out the test article with another material is also needed. Measuring the temperature profile across the thickness of the insulation is needed to characterize and understand the performance of the insulation system. Furthermore, the ability to vary the vacuum level from high vacuum to soft vacuum to atmospheric pressure is needed. This vacuum level should be maintained very steadily for long periods of time and be measured very accurately.
In view of the foregoing background, it is therefore an object of the invention to provide reliable and accurate testing of continuously rolled thermal insulation materials to measure the temperature profile across the thickness of the insulation and determine the apparent thermal conductivity thereof.
This and other objects, features and advantages in accordance with the present invention are provided by a method for testing thermal insulation in a cryostatic insulation tester comprising a vacuum chamber and a cold mass including a test chamber and upper and lower guard chambers adjacent thereto. The method includes positioning the thermal insulation within the vacuum chamber and adjacent the cold mass, supplying cryogenic liquid to the test chamber, upper guard and lower guard to create a first gas layer in an upper portion of the lower guard chamber and a second gas layer in an upper portion of the test chamber, and sensing temperatures within the vacuum chamber to test the thermal insulation.
Supplying the cryogenic liquid preferably includes continuously replenishing the cryogenic liquid to the test chamber, upper guard and lower guard until a desired vacuum level and temperatures within the vacuum chamber reach a substantially steady state, stopping the flow of the cryogenic liquid to the test chamber to create the second gas layer in the upper portion of the test chamber, and stopping the flow of the cryogenic liquid to the lower guard chamber to create the first gas layer in the upper portion of the lower guard chamber. The method may also include measuring a boil-off gas flow rate of the cryogenic liquid from the test chamber until the boil-off gas flow rate is substantially stable.
A cold boundary temperature (CBT) is defined between the insulation material and the cold mass, and a warm boundary temperature (WBT) is defined at an outer surface of the insulation material. The performance of the insulation material is preferably measured when the CBT, WBT, and temperatures of the cold mass and vacuum chamber are stable. The apparent thermal conductivity value (k) of the insulation material is measured from the measured boil-off gas flow rate, a difference between CBT and WBT, latent heat of vaporization, and the inner and outer diameters of the insulation material and effective heat transfer length of the test chamber.
The cold mass preferably includes a cylindrical cold mass, and the thermal insulation may include continuously rolled thermal insulation. Furthermore, positioning the thermal insulation within the vacuum chamber and adjacent the cold mass may include installing the continuously rolled thermal insulation around the cylindrical cold mass, enclosing the cold mass having the continuously rolled thermal insulation material installed thereon with a vacuum can and base plate, and adjusting vacuum pressure inside the vacuum chamber to a desired vacuum level.
Installing the continuously rolled thermal insulation around the cylindrical cold mass may comprise placing temperature sensors between various layers of the continuously rolled insulation material. Also, installing the continuously rolled thermal insulation around the cylindrical cold mass may include wrapping the continuously rolled thermal insulation around a cylindrical sleeve, and sliding the cylindrical sleeve over the cold mass. A gap between the sleeve and the cold mass is preferably less than 1 mm.
Furthermore, the desired vacuum level in the vacuum chamber is between 10xe2x88x921 torr and 760 torr (atmospheric pressure). The temperature of the vacuum can is maintained at between approx 273 K and 373 K, and the temperature of the cold mass is maintained at approximately the normal boiling point of the cryogenic liquid (approximately 77.8 K for LN2). Cryogenic liquids may include one of liquid nitrogen, argon, oxygen, hydrogen, helium and methane.
Objects, features and advantages in accordance with the present invention are also provided by a cryostatic insulation tester including a vacuum chamber, and a cold mass within the vacuum chamber for being positioned adjacent thermal insulation being tested. The cold mass comprises a test chamber and upper and lower guard chambers adjacent thereto. A cryogenic liquid supply system is connected to the test chamber, upper guard and lower guard to create a first gas layer in an upper portion of the lower guard chamber and a second gas layer in an upper portion of the test chamber. Also, a plurality of temperature sensors are within the vacuum chamber.
The cryogenic liquid supply comprises pipes, valves and sensors to continuously replenish the cryogenic liquid to the test chamber, upper guard and lower guard until a desired vacuum level and temperatures within the vacuum chamber reach a substantially steady state, stop the flow of the cryogenic liquid to the test chamber to create the second gas layer in the upper portion of the test chamber, and stop the flow of the cryogenic liquid to the lower guard chamber to create the first gas layer in the upper portion of the lower guard chamber. A vacuum pumping system is preferably included for creating a desired vacuum level in the vacuum chamber between 10xe2x88x927 torr and 760 torr. Also, a heater for maintaining a temperature of the vacuum can at between approx 273 K and 373 K may be provided.
Another aspect of the invention is a method for testing thermal insulation in a cryostat insulation tester comprising a vacuum chamber and a cold mass, including controlling a thermal coupling between the cold mass and the thermal insulation to set an elevated cold boundary temperature substantially greater than a temperature of the cryogenic liquid; and sensing temperatures within the vacuum chamber to test the thermal insulation with respect to the elevated cold boundary temperature. Positioning the thermal insulation preferably comprises installing the thermal insulation on a sleeve and sliding the sleeve over the cold mass. Controlling the thermal coupling preferably includes setting a spacing between the sleeve and cold mass. Such a gap may be between approximately 1 mm and 25 mm, for example. Controlling the thermal coupling may also include installing gap filler material (e.g. vacuum grease) between the cold mass and the sleeve, or forming the sleeve with at least one of predetermined heat transfer characteristics (thermal conductance) and a predetermined thickness. Any combination of gap spacing, sleeve material, sleeve thickness and filler material may be used to provide the desired elevated CBT.